The present invention relates to a continuously variable transmission unit incorporating a toroidal-type continuously variable transmission that is utilized for a power transmission system of an automobile, for example, and more particularly, to a continuously variable transmission unit reduced in size and improved to secure the durability of the toroidal-type continuously variable transmission.
A study is made of use of a toroidal-type continuously variable transmission schematically shown in FIGS. 4 and 5 as an automotive transmission. In this continuously variable transmission, an input disc 2 is supported coaxially on an input shaft 1. An output disc 4 is fixed to an end portion of an output shaft 3 that is coaxial with the input shaft 1. Pivots 5 and trunnions 6 are arranged in a casing (not shown) that contains the transmission therein. The pivots 5 are situated in torsional positions with respect to the input and output shafts 1 and 3. The trunnions 6 are rockable around their corresponding pivots 5.
Each pivot 5 is attached to each side face of each corresponding trunnion 6 in a coaxial manner. A displacement shaft 7 is provided in the center of each trunnion 6. When each trunnion 6 rocks around its corresponding pivot 5, the angle of inclination of its corresponding displacement shaft 7 changes. The displacement shaft 7 on each trunnion 6 supports a power roller 8. The roller 8 can rotate around the shaft 7. Each power roller 8 is interposed between opposite inner side faces 2a and 4a of the input and output discs 2 and 4. The inner side faces 2a and 4a are concave surfaces that can be obtained if an arc of a circle around each pivot 5 is rotated around the shafts 1 and 3. An outer peripheral surface 8a of each power roller 8 is a spherical convex surface that mates with the concave surfaces. The outer peripheral surface 8a is in contact with the inner side faces 2a and 4a of the discs 2 and 4.
A loading cam device 9 for use as pressure means is interposed between the input shaft 1 and the input disc 2. The cam device 9 elastically presses the input disc 2 toward the output disc 4. The rotation of the input shaft 1 is transmitted to the input disc 2 via the device 9. The loading cam device 9 includes a loading cam (cam plate) 10, which can rotate integrally with the input shaft 1, and a plurality of rollers 12 (e.g., four in number) that are held for rolling motion by means of a ring-shaped retainer 11. A cam face 13 that undulates in the circumferential direction is formed on one surface (right-hand surface in FIG. 4) of the loading cam 10. A cam face 14, which resembles the cam face 13 in shape, is formed on the outer side face (left-hand surface in FIG. 4) of the input disc 2. The rollers 12 are rotatably supported by shafts that extend radially from the center of the input shaft 1.
In the toroidal-type continuously variable transmission constructed in this manner, the loading cam 10 rotates as the input shaft 1 rotates. When the cam 10 rotates, its cam face 13 presses the rollers 12 toward the cam face 14 of the input disc 2. In consequence, the input disc 2 is pressed against the power rollers 8, and at the same time, the cam faces 13 and 14 push each other with the rollers 12 between them, whereupon the input disc 2 rotates. As the input disc 2 rotates, the power rollers 8 rotate around their corresponding shafts 7. The rotation of each roller 8 is transmitted to the output disc 4. As the output disc 4 rotates, the output shaft 3 that is fixed to the disc 4 rotates.
The following is a description of the way of changing the ratio (i.e., gear ratio) of the rotating speed of the output shaft 3 to that of the input shaft 1. In decelerating the rotation of the input shaft 1 and transmitting it to the output shaft 3, the trunnions 6 are tilted around their corresponding pivots 5, as shown in FIG. 4. Thus, each displacement shaft 7 is inclined so that the outer peripheral surface 8a of each power roller 8 is in contact with the central portion of the inner side face 2a of the input disc 2 and the outer peripheral portion of the inner side face 4a of the output disc 4. In accelerating the rotation of the input shaft 1 and transmitting it to the output shaft 3, in contrast with this, the trunnions 6 are tilted in the opposite direction around their corresponding pivots 5, as shown in FIG. 5. Thus, each displacement shaft 7 is inclined so that the outer peripheral surface 8a of each power roller 8 is in contact with the outer peripheral portion of the inner side face 2a of the input disc 2 and the central portion of the inner side face 4a of the output disc 4. If each displacement shaft 7 is inclined at an angle intermediate between the ones shown in FIGS. 4 and 5, an intermediate gear ratio can be obtained between the input and output shafts 1 and 3.
FIGS. 6 and 7 show a more specific example of the toroidal-type continuously variable transmission. In this example, the input disc 2 and the output disc 4 are rotatably supported around a cylindrical input shaft 15 by means of needle bearings 16, individually. A through hole 17 having a circular cross section is formed in the central portion of each of the discs 2 and 4. The holes 17 are formed extending in the axial direction of the input shaft 15 through the respective inner side faces 2a and 4a and outer side faces of the discs 2 and 4. Each needle bearing 16 is provided between the inner peripheral surface of its corresponding through hole 17 and the outer peripheral surface of an intermediate portion of the input shaft 15. A retaining groove 18 is formed on the inner peripheral surface of an end portion of each hole 17. A snap ring 19 is fitted in each retaining groove 18. The rings 19 in the grooves 18 prevent the needle bearings 16 from slipping out of through holes 17 toward the inner side faces 2a and 4a of the discs 2 and 4. The loading cam 10 is mounted on an end portion 15c (left-hand end portion in FIG. 6) of the input shaft 15 by spline fitting. A flange portion 20 prevents the cam 10 from moving away from the input disc 2. The loading cam 10 and the rollers 12 constitute the loading cam device 9, which rotates the input disc 2 while pressing it toward the output disc 4 as the input shaft 15 rotates. An output gear 21 is coupled to the output disc 4 by means of a key 22. Thus, the disc 4 and the gear 21 can rotate in synchronism with each other.
As shown in FIG. 7, the opposite end portions of the trunnions 6 are supported by means of a pair of support plates 23. The trunnions 6 are rockable around their corresponding pivots 5 and movable in the axial direction (horizontal direction in FIG. 7) of the pivots 5. Each displacement shaft 7 is inserted in a circular hole 24 that is formed in the central portion of each trunnion 6. Each shaft 7 includes a support shaft portion 25 and a pivot portion 26 that extend parallel and eccentrically to each other. The support shaft portion 25 is rotatably supported in each trunnion 6 by means of a radial needle bearing 27 that is fitted in the hole 24. Each power roller 8 is rotatably supported on its corresponding pivot portion 26 by means of a radial needle bearing 28.
The paired displacement shafts 7 are located diametrically opposite to each other with respect to the input shaft 15. The pivot portions 26 are eccentric to their corresponding support shaft portions 25 in the same direction with respect to the rotating direction of the discs 2 and 4. The direction of their eccentricity is substantially perpendicular to the axial direction of the input shaft 15. Accordingly, each power roller 8 can move for a certain distance in the axial direction of the input shaft 15. The power rollers 8, movable in this manner, are allowed to shift their positions in the axial direction of the input shaft 15 even if the discs 2 and 4, power rollers 8, etc. are elastically deformed by substantial loads that act thereon during torque transmission. Thus, those components can avoid being subjected to excessive forces.
A thrust ball bearing 29 and a thrust needle bearing 30 are interposed between each power roller 8 and its corresponding trunnion 6. The ball bearing 29 supports a thrust load on the power roller 8 and allows the roller 8 to rotate. The needle bearing 30 supports a thrust load from the power roller 8 that acts on an outer race 31 of the ball bearing 29. Further, the bearing 30 allows the pivot portion 26 and the outer race 31 to rock around the support shaft portion 25.
A driving rod 32 is coupled to one end portion (left-hand end portion in FIG. 7) of each trunnion 6. A driving piston 33 is fixed to the outer peripheral surface of an intermediate portion of each rod 32. Each piston 33 is stored liquid-tight in a driving cylinder 34.
In the toroidal-type continuously variable transmission constructed in this manner, the rotation of the input shaft 15 is transmitted to the input disc 2 through the loading cam device 9. The rotation of the input disc 2 is transmitted to the output disc 4 through the power rollers 8. The rotation of the output disc 4 is transmitted to the output gear 21. In changing the ratio between the respective rotating speeds of the input shaft 15 and the output gear 21, the paired driving pistons 33 are displaced in opposite directions. As the pistons 33 are displaced in this manner, the paired trunnions 6 are displaced in opposite directions. In FIG. 7, for example, the lower power roller 8 moves to the right, and the upper power roller 8 to the left. As a result, the directions of tangential forces that act on the regions where the respective outer peripheral surfaces 8a of the power rollers 8 and the inner side faces 2a and 4a of the discs 2 and 4 are in contact with one another change. As the directions of those forces change in this manner, the trunnions 6 tilt in opposite directions around their corresponding pivots 5. In consequence, as in the case shown in FIGS. 4 and 5, the positions of contact between the respective outer peripheral surfaces 8a of the power rollers 8 and the inner side faces 2a and 4a of the discs change, so that the speed ratio between the input shaft 15 and the output gear 21 changes.
As power is transferred between the input shaft 15 and the output gear 21, the contact regions between the components are subjected to some elastic deformation by transmitted loads. Since each power roller 8 is displaced in the axial direction of the input shaft 15 in response to the elastic deformation, the displacement shaft 7 that supports the roller 8 slightly rotates around its corresponding support shaft portion 25. AS this is done, the outer race 31 of each thrust ball bearing 29 and its corresponding trunnion 6 are displaced relatively to each other. Since the thrust needle bearing 30 is interposed between the outer race 31 and the trunnion 6, the relative displacement requires only a small force. Thus, the angle of inclination of each displacement shaft 7 can be changed with a small force.
Described in Jpn. Pat. Appln. KOKAI Publication Nos. 1-234646, 7-158711, 8-21503, and 8-35549 are toroidal-type continuously variable transmissions of the double-cavity type that have been developed to increase torque to be transmitted from an input shaft to an output shaft. As shown in FIGS. 8 and 9, one such double-cavity transmission comprises two input discs 2A and 2B and two output discs 4 that surround an input shaft 15a. The discs 2A, 2B and 4 are arranged in parallel with one another with respect to the direction of power transmission.
In the toroidal-type continuously variable transmission shown in FIGS. 8 and 9, an output gear 21a is provided on an intermediate portion of the input shaft 15a. The output gear 21a is rotatable with respect to the input shaft 15a. A cylindrical sleeve 35 is provided in the central portion of the output gear 21a. The two output discs 4 are fixed individually to the opposite end portions of the sleeve 35 by spline fitting. A needle bearing 16 is provided between the inner peripheral surface of a through hole 17 in the central portion of each output disc 4 and the outer peripheral surface of the input shaft 15a. The bearing 16 supports each output disc 4 and the output gear 21a for rotation around the input shaft 15a and movement in the axial direction. The input discs 2A and 2B on the opposite end portions of the input shaft 15a can rotate together with the shaft 15a. The input shaft 15a is rotated by means of a driving force transmitted from a drive shaft 36 on the left-hand side of FIG. 8 through the loading cam device 9. A radial bearing 37 is provided between the outer peripheral surface of the distal end portion of the shaft 36 and the inner peripheral surface of the proximal end portion of the input shaft 15a. A sliding bearing or needle bearing is used as the bearing 37. The radial bearing 37 connects the drive shaft 36 and the input shaft 15a so that they can be relatively displaced for a certain distance in the rotating direction without changing their coaxial relation.
A coned disk spring 39, which can generate a relatively great repulsive load, is interposed between the back surface of the first input disc 2A on the right-hand side of FIG. 8 and a loading nut 38 on the input shaft 15a. The nut 38, aided by the spring 39, restrains the input disc 2A from being displaced relatively to the shaft 15a in the axial direction. The second input disc 2B that faces the loading cam 10 is allowed, by a ball spline 40, to be displaced relatively to the input shaft 15a in the axial direction. A retaining step portion 41 is formed on the outer peripheral surface of an intermediate portion (near the ball spline 40) of the input shaft 15a. A coned disk spring 42 is interposed between the step portion 41 and an end face of the input disc 2B. The spring 42 generates a repulsive load smaller than the one generated by the spring 39. The repulsive load from the spring 39 applies a pilot pressure to the contact regions between the inner side faces 2a and 4a of the discs 2A, 2B and 4 and the respective outer peripheral surfaces 8a of the power rollers 8. Based on this pilot pressure, a contact pressure can be applied to the contact regions even when the loading cam device 9 produces no thrust or only a small thrust. Thus, the toroidal-type continuously variable transmission can transmit even low torque.
The output gear 21a is supported in an intermediate wall 43 inside the housing by means of angular ball bearings 44 so as to be rotatable and immovable in the axial direction. In the toroidal-type continuously variable transmission of the double-cavity type, at least one of the input discs 2A and 2B is allowed to move in the axial direction of the input shaft 15a by means of the ball spline 40. This is done in order that the input discs 2A and 2B, which are rotatable in synchronism with each other, can move for a certain distance in the axial direction of the input shaft 15a in response to the elastic deformation of the components that is caused by the force of pressure from the loading cam device 9.
When the toroidal-type continuously variable transmission of the double-cavity type with the above-described construction is operated, the rotation of the drive shaft 36 is transmitted to the second input disc 2B through the loading cam device 9. As the rotation of the input disc 2B is transmitted to the first input disc 2A through the input shaft 15a, the two input discs 2A and 2B rotate synchronously. The respective rotations of the input discs 2A and 2B are transmitted individually to their corresponding output discs 4 via the power rollers 8. In consequence, the sleeve 35 that is in spline-engagement with the output discs 4 rotates. As the sleeve 35 rotates, the output gear 21a rotates. Thus, in the toroidal-type continuously variable transmission of the double-cavity type, the torque transferred from the drive shaft 36 to the output gear 21a is transmitted through two power transmission lines (two input discs 2A and 2B and two output discs 4) that are arranged in parallel with each other. Accordingly, the double-cavity transmission can transmit higher torque than a toroidal-type continuously variable transmission of the single-cavity type (shown in FIGS. 4 and 5). In the double-cavity transmission, moreover, the gear ratios between the discs 2A, 2B and 4 can be changed in synchronism with each other by synchronously changing the angle of inclination of the power roller 8 between the one input disc 2A and its corresponding output disc 4 and that of the power roller 8 between the other input disc 2B and its corresponding output disc 4.
In the case where toroidal-type continuously variable transmission is incorporated in an actual automobile, a toroidal-type continuously variable transmission 47 and an epicyclic train 50 may be combined in the manner shown in FIG. 10. In the continuously variable transmission unit that combines the toroidal-type transmission 47 and the epicyclic train 50, a drive shaft (crankshaft) 46 of an engine 45 for use as a drive source is connected to an input shaft of the transmission 47. The transmission 47 is constructed in the same manner as the one shown in FIGS. 6 and 7. An output shaft 49 for rotating the driving wheels of the automobile is coupled to a sun gear that constitutes part of the epicyclic train 50. The output shaft 49 rotates integrally with the sun gear. Conventional differential gears are arranged between the output shaft 49 and the driving wheels.
The output discs of the toroidal-type continuously variable transmission 47 and members that constitute part of the epicyclic train 50 are connected by means of a first power transmission device 52 so that they can transmit turning effort. The input shaft of the transmission 47 and the drive shaft 46 are connected to the other members of the epicyclic train 50 by means of a second power transmission device 53 so that they can transmit turning effort. Further, the continuously variable transmission unit is provided with switching means for switching the state of transmission between the drive shaft 46 and the output shaft 49 to a high-speed drive mode, low-speed drive mode, or reverse mode. In this continuously variable transmission unit, the ratio (.beta./.alpha.) between a reduction ratio a of the first power transmission device 52 and a reduction ratio .beta. of the second power transmission device 53 is made substantially equal to a reduction ratio (reduction ratio between the input and output shafts 1 and 3 in the state shown in FIG. 5) i.sub.H for the maximum acceleration of the toroidal-type continuously variable transmission 47.
The continuously variable transmission unit shown in FIG. 10 is of the power-split type as it is called. The transmission unit of this type is designed so that all the power (torque) applied to the drive shaft 46 is transmitted to the output shaft 49 through the toroidal-type continuously variable transmission 47 in the low-speed drive mode. In the high-speed drive mode, on the other hand, the power applied to the drive shaft 46 is transmitted to the output shaft 49 through the epicyclic train 50, and is partially applied to the output discs of the transmission 47 through the epicyclic train 50. Thus, the driving force of the engine 45 is transmitted to the output shaft 49 through only the transmission 47 in the low-speed drive mode. In the high-speed drive mode, the driving force of the engine 45 is transmitted to the output shaft 49 by means of the epicyclic train 50. By doing this, the torque applied to the transmission 47 in the high-speed drive mode can be lessened to improve the durability of the components of the transmission 47, and the torque transfer efficiency of the continuously variable transmission unit can be improved as a whole.
However, the conventional continuously variable transmission unit of the power-split type cannot always efficiently transmit high power. In a continuously variable transmission unit described in Jpn. Pat. Appln. KOKAI Publication No. 1-169169, for example, a toroidal-type continuously variable transmission of the single-cavity type is combined with two epicyclic trains. Therefore, the conventional transmission unit is large-sized and complicated in construction, and cannot efficiently transmit high torque for its heavy weight. A continuously variable transmission unit described in Jpn. Pat. Appln. KOKAI Publication No. 1-312266 is subject to the same problem.
Described in Jpn. Pat. Appln. KOKAI Publication No. 9-89072 is a continuously variable transmission unit of the so-called geared-neutral type, which combines a toroidal-type continuously variable transmission and an epicyclic train. The transmission unit of this type is designed so the that driving force is transmitted through the epicyclic train and the transmission in the low-speed drive mode and through only the transmission in the high-speed drive mode. In the case of the transmission unit of this geared-neutral type, the toroidal-type continuously variable transmission is subjected to high torque during the period from the start of drive to the low-speed drive mode. Accordingly, the transfer efficiency is low, and it is hard to reconcile the durability and compactness of the components of the transmission. A continuously variable transmission unit described in Jpn. Pat. Appln. KOKAI Publication No. 10-103461 is subject to the same problem.